rebase: sort destmap topologically Previously rebase source and destination could not overlap. But with the multi-destination support, source and destination could reasonably partially overlap. That requires another topological sort on `{sourcerev: destrev}` graph (destmap). This patch implements that. If a revision's destination is itself, the error message gets changed from "source is ancestor of destination" to "source and destination form a cycle". Not marking as BC since automation should depend on exit code, not error message. Differential Revision: https://phab.mercurial-scm.org/D470